This invention relates to a collapsible depth controller for mounting in relation to seismic cables or similar, comprising two wings, each being rotatably coupled to the main body relative to a first rotational axis perpendicular to the longitudinal direction of the depth controller, to be foldable into an suitable room in the main body of the depth controller.
In towing a seismic cable, e.g. a hydrophone cable, in connection seismic studies it is important to control the level of the cable relative to the water surface and the bottom. Deviations from the correct position may result in inaccurate measurements, an in the worst case erroneous measurements, which may result in drilling for oil resources in wrong places, or in not drilling at all.
A number of different depth controllers are known. One example is shown in Norwegian patent application no. 96.0506, which is especially aimed at a wing construction for optimizing the lift. They are effective, but are impractical when the cable is wound in or laid out. When a cable is wound in, e.g. in bad weather or for maintenance, the winches must be stopped and the depth controller removed before the winding continues.
With a system comprising eight cables, each with sixteen depth controllers, and where it takes 30 seconds to remove each depth controller, it will take up to one hour extra to take the system in. Correspondingly the operation of laying the cable out will take twice as long, as each depth controller must be tested. The use of a depth controller that does not have to be removed from the cable will thus save a large amount of time. In bad weather a fast withdrawal of a seismic cable may also avoid damages on the equipment mounted on the cable.
When letting the cable out the wings may be unfolded after the depth controller has left the aft of the vessel, so that damages on the wings are avoided. Correspondingly, the wings may be temporarily folded when there is a danger for collisions between the cables or collisions with stray object found at sea, which may damage the depth controller. U.S. Pat. No. 3.943.483 describes a collapsible depth controller with wings which may be unfolded to provide a lift. A problem with the solution described in this patent is that the mechanism for controlling the lift of the depth controller is complicated, and thus sensitive to damages. The depth controller in the US patent comprises two sets of wings for positive and negative lift, respectively. The wings are unfolded using a piston rod which through hinges is connected to rods which in turn pushes the wings. The wings are rotatably coupled to the depth controller in one of their ends.
In U.S. Pat. No. 3.961.303 a similar solution is shown in which the negative lift may be adjusted, but which comprises a constant positive lift.
Both the abovementioned solutions are mechanical (pneumatic) and are folded over the cable on suitable fastening devices. Therefore they are relatively large and are not suited to stay on the cable. The need for two different wings for adjusting the position under the sea surface make the solutions unnecessarily complicated, and does not provide the same possibility for varying the lift an conventional depth controllers.
It is thus an object of this invention to provide a robust, collapsible depth controller being sufficiently compact to be laid out or pulled in without being removed from the cable, or which when in use is as flexible as conventional depth controllers. This is obtained using a depth controller as described above, being characterized in that the depth controller comprises a first motor for rotation of one at least partially cylindrically shaped cam plate about its axis, the axis essentially corresponding to the longitudinal axis of the depth controller, said cam plate (mushroom cam) comprising at least one trace or cam which in at least part of its length is slanting relative to the axis of the cam plate, at least one cam follower engaging into said cam plate an comprising a first coupling device for rotatable coupling to the wings, said first coupling device being positioned at a chosen distance from the first rotational axis of the wing.
In an especially preferred embodiment each wing is also rotatably coupled to the depth controller relative to a second axis being parallel to the longitudinal direction of the wing, and that the wing at a chosen distance from this axis is coupled to a second cam follower engaging in the cam plate to provide for a possible rotation of the wing relative to the second axis by rotating the cam plate. This way a possibility is provided for controlling the angle of the wings, and thus their lift, using the same cam plate. Thus a flexible and simple solution is provided with good possibilities for adjusting the lift.
When the wings are unfolded it is usually necessary to adjust the orientation of the wings, so that they are unfolded in the horizontal plane. This is solved according to a preferred embodiment of the invention in that the depth controller comprises a coaxially positioned core firmly connected to the cable, and a second motor for rotating the main body of the depth controller relative to the core. The orientation of the depth controller may be measured using a gravimeter or similar.
The invention will be described more closely referring to the enclosed drawings, which show one of many possible embodiments of the invention.